Today, many elevator drives with induction motors use the principle of vector control. Vector control leads to high dynamic behavior of the motor and therefore guarantees a good riding comfort and short flight times because the elevator velocity follows its reference value without any deviation.
To obtain the advantages of this control method, it is necessary to know the electrical parameters of the controlled motor. The main idea behind vector control is to control torque current independently from control of flux in the stator. This independence of control can only occur if the rotor time constant value used is correct. See "The Influence of Motor Parameter Deviations in Feedforward Field Orientation Drive Systems," K. B. Nordin, D. W. Novotny, D. S. Zinger, 1984 IEEE IAS84: 22B, p. 525. Unfortunately, the rotor time constant is not constant. It depends on the rotor resistance and therefore on the rotor temperature which varies according to the load conditions of the elevator. There exist many general algorithms to estimate the rotor time constant, but they are usually not suitable to elevator drives. Either these methods are very complicated and need expensive hardware or they use input and output values which are not available for elevator drives.
One class of algorithms uses special noise signals which are applied to the motor. The voltages and currents and the velocity of the motor are measured and the rotor time constant is calculated out of the measurement results. These algorithms are not suitable to elevators. It is not possible to feed the motor with noise signals because of the resulting jerk and poor riding comfort. See, for example, "Microprocessor Control of Induction Motor," R. Gabriel and W. Leonhard, IEEE 1982.
Another class of algorithms for estimating rotor time constant uses special operation modes, for example, idle running or special velocities which are not available for elevators. See, for example, "On the Identification of Induction Motor Parameters," M. Song, J. Mescua, Proc. of the 18th Southeastern Symposium on System Theory, IEEE, Knoxville, USA, April 1986.
See also "Self Commissioning--A Novel Feature of Modern Inverter-Fed Induction Motor Drives," H. Schierling, page 287, Proc. of Third International Conference on Power Electronics and Variable Speed Drive, IEEE, London, July, 1988. This reference discusses the use of stator voltage to determine rotor time constant. The problem, however, is that in elevators for reasons of cost, stator voltage is not usually measured and therefore these algorithms are useless.